Traditionally, fixed or floating platforms, or floating production vessels, are used in the offshore oil and gas production. In the operation of offshore platforms, it can be necessary to install electrical equipment under water, e.g. for controlling functions of a subsea Christmas tree or a subsea blowout preventer. More recently, processing facilities are being relocated to the ocean floor. Installations on the ocean floor can comprise a range of components, including pumps, compressors and the like which require electric power for operation. Power supply can occur through a subsea power grid installed on the ocean floor, which may for example comprise a subsea transformer, a subsea switchgear and a subsea variable speed drive for powering the above mentioned subsea loads. It needs to be ensured that the installed equipment operates reliably even under the high pressure that prevails at the rated installation depth which can be 3,000 m or more.
To protect the equipment from the corrosive environment of the surrounding seawater and to deal with the high pressures, two different solutions were proposed. A pressure resistant enclosure can be provided, which has a close to atmospheric internal pressure, enabling the use of conventional electric and electronic components therein. Such enclosures need to have relatively thick walls and are thus bulky and heavy, since they have to withstand high differential pressures.
Another solution is the use of pressurized (or pressure compensated) enclosures, which employ a pressure compensator that balances the pressure in the enclosure to the pressure prevailing in the ambient seawater.
The pressurized enclosure is generally filled with a liquid, and components operated inside the pressurized enclosure are made to be operable under high pressures. The pressure compensator balances the pressure and compensates variations in the volume of the liquid filling the enclosure, which may for example occur due to variations in outside pressure and/or temperature. Temperature changes can also be caused by internal heating, e.g. by electric losses of components provided inside the enclosure of the subsea device. The corresponding volume increase of the liquid filling the enclosure may then be taken up by the pressure compensator, which is thus also termed volume compensator.
Pressure compensators may include bellows, bladders, pistons, membranes or the like. For some types of bellows, such as formed bellows, the stroke length of the bellows can be limited, resulting in a relatively low utilization factor of the volume of the compensator system, and thus in a large ‘dead volume’. Furthermore, the liquid filling such pressure compensator needs to be compensated itself (i.e. changes of its volume due to temperature/pressure changes need to be taken up by the compensator). Such compensator systems can thus be relatively large and heavy.
The document EP 2610881 B1 discloses a pressure compensator that reduces the dead volume, i.e. the liquid volume inside the pressure compensator that additionally needs to be compensated.
Other types of bellows may have a smaller dead volume and an increased compensation capacity. Yet such bellows may be more vulnerable to corrosion and leakage. The bellows of a pressure compensator is exposed to the subsea environment, in particular to the seawater. This may cause corrosion problems for the bellows and may lead to the ingress of seawater into the enclosure of the subsea device upon failure of the bellows. Such seawater ingress may lead to a complete failure of the subsea device, e.g. due to short circuit currents or the like.
It is desirable to provide a pressure compensator for use with a subsea device that can be manufactured easily and cost efficiently. It is further desirable that the pressure compensator is reliable during operation and has a long lifetime. It is desirable to reduce the size of pressure compensators, and to increase the utilization factor and compensation capacity. Also, it is desirable that the pressure compensator is protected from corrosion and provides protection against seawater ingress.